To facilitate the transportation of perishable goods, cargo vessels such as, for example, truck trailers and/or containers include a trailer refrigeration unit (TRU) to maintain the temperature of the cargo compartment at the desired temperature. Products at risk of damage from elevated ambient temperature may be maintained at “room temperature” (approximately 20° C./68° F.), while refrigerated goods are typically held slightly above the freezing point (1° C./35° F.), and frozen products may be stored at a variety of temperature set points between −15° C./0° F. and −30° C./−20° F. The cargo may contain a variety of consumer products such as produce, frozen and refrigerated meat, dairy, candy, pharmaceuticals, and flowers.
Contemporary TRUs with diesel-powered compressor systems provide the required cooling performance, but are becoming less attractive from an economic standpoint due to the cost of maintenance, fueling, and regulatory compliance. Additionally, society places substantial pressure on the users of TRUs to reduce environmental impact, particularly in urban areas where noise and exhaust emissions are perceived to be especially problematic.
To provide an alternative method of cargo cooling with fewer environmental drawbacks than diesel TRUs, a cryogenic cooling system (CCS) utilizes the stored thermal energy of liquid nitrogen (LN2) for the purpose of maintaining cargo temperature. Two general types of CCS may be employed—“direct” systems where the LN2 is dispersed directly into the cargo compartment, and “indirect” systems that use an evaporator (similar to that of compressor-based system) and circulating fans to transfer heat from the cargo into the cooling medium.
The direct method of injecting LN2 is preferred due to its superior efficiency as compared to indirect systems—nearly all of the potential heat energy absorption of the cooling medium is realized through direct interaction between the LN2 and the cargo, and no defrost cycles are required to remove ice from an evaporator (icing can occur whenever the evaporator surface temperature is lower than the freezing point of water and moisture is present in the air). Since fuel is necessarily limited by the size of the LN2 vessel and thus efficiency is an important parameter in maximizing the utility of this system, direct systems can have a significant advantage in energy efficiency. Additionally, as direct LN2 systems do not require the use of circulation fans during the cooling cycle, they have lower electrical power consumption during the cooling cycle (important as the trailer is often not connected to a tractor or shore power during the lengthy loading and staging process), substantially lessened noise emissions, and) and decreased drying of food.
Some refrigerated vehicles may include direct LN2 systems that distribute LN2 through a spray tube arrangement, and simultaneously through an evaporator arrangement. The LN2 flows through the evaporator arrangement and is converted to gaseous nitrogen which is used to drive a fan for drawing air into the space to achieve environmental control. However, the introduction of liquid nitrogen directly into the enclosed trailer space presents a safety risk to operators because of the possibility of entering an atmosphere that contains dangerous levels of gaseous nitrogen. For example, as the liquid nitrogen is sprayed within the space, air is drawn into the space, however, serious safety risks are present due to the significant amount of gaseous nitrogen occupying the space.
Accordingly, new features are necessary to improve the safety and monitoring of refrigerated vehicles that include cryogenic cooling systems to reduce the potential health risks exposed to a vehicle operator when operating the refrigerated vehicles and loading and unloading products from the refrigerated trailers. The present invention is directed to satisfying these needs.